Heating elements commonly utilize metal resistors containing iron, chromium and aluminum alloys. These resistors are commonly constructed in a helical form and are mounted within a tube formed of quartz, vitreous silica, or vitrified ceramic composition.
Heating elements such as those described above have many uses in industry. They are, in particular, very widely used for drying papers, fabrics, paints, wood, agglomerated or stratified plates and panels and various granulates. Frequently they are installed in heating containers, wherein they are typically arranged parallel to one another in front of a reflecting metal plate.
Radiant heating elements in which the active element comprises a helical resistor exposed to air, have a limited operating temperature, e.g., 1000.degree. C . The mechanical resistance of the metal alloy used in the resistor, the effects of oxidation, and the desired wavelength of operation all restrict the operating temperature. An extreme operating temperature is associated with short wavelength emission, and thus the medium wavelength is the desired energy form. Since lower operating temperatures are used to produce medium wavelength radiation, in order to produce enough thermal energy for practical use it is desirable to utilize several resistor filaments positioned parallel to one another. Most commonly, two such resistors are utilized.
Moreover, it is common to install the two resistors parallel to each other in a single tube, such as a silica based tube. Two methods have been utilized in the prior art for separating two resistors housed within the same tube. In the first method, the tube was divided into hemi-sections (i.e., with a cross-section shaped like the figure "8") by a septum running the length of the tube. In a second method, a second, smaller tube is placed as an insulator through the center of the main tube. This tube may, for example, be formed from the same material as the main tube.
One problem with the use of the first method is the necessity of utilizing electrical connections at both ends of the tube. The second system, by contrast, could allow sufficient room between the smaller tube and the wall of the main tube to permit such a connection at the ends of both resistors. It is also possible to pass an electric cable through the smaller tube so as to make a connection with the resistors at the end of the main tube and thus to obviate the need for electrical connections at either end of the heating element. However, even though it might be possible to utilize a form of tube within a tube design to gain a single end electrical connection heating element, the system has the drawback of creating a vibratory phenomena during actual operation.
Depending on the characteristics of each filament, such as diameter, length, temperature, and wavelength, it may be advantageous to supply electrical power to two resistors in either a parallel or a series circuit configuration. In the prior art embodiment discussed above having a cross-section shaped like the figure "8", it will be necessary to make connections to the heating element at either end of its tube. Junction boxes can be connected to the filaments at either end of the tubes to allow either series or parallel configuration.
If the heating element design utilized is the small tube within the main tube embodiment, it might be possible to make all electrical connections from one end of the tube only by passing a connecting wire behind the smaller tube in order to connect to the ends of the resistors distal to the electrical terminus of the heating element. The disadvantage to this arrangement is that once the heating element is configured for either parallel or series circuitry, changing the configuration requires disassembly of the heating element. There are no known heating elements in the prior art comprising a heating element tube with all the electrical connections at one end only, in which the configuration of multi-resistors within the element could be changed easily without the need to disassemble the heating element.
In the prior art, heating elements utilizing the vitreous silica tubes described above have been provided with a thin layer of gold on their rear face. This technique effectively diminishes the amount of thermal radiation released at the rear aspect of the heating element. This technique has been less than satisfactory, however, in that the gold layer is mechanically fragile and its presence necessarily limits the operating temperature of the heating element. If any overheating of the silica/gold interface occurs i.e., if the temperature exceeds 800.degree. C., the gold layer becomes virtually useless and no longer exerts any effect upon the radiation. Furthermore, prior to the installation of the heating element tube, the gold layer is extremely susceptible to mechanical damage. Moreover, utilizing gold in a heating element also significantly adds to the expense associated with this item.
Other means have been used in the past for reflecting and concentrating radiation emitted by the incandescent filaments of heating elements. U.S. Pat. No. 4,001,622 discloses, for example, a high-temperature linear filament of tungsten placed eccentrically towards a rear wall of a circular tube of quartz and parallel to the tube axis. The tube itself is embedded in a coaxial semi-cylinder of ceramic fibres in such a manner as to focus the radiation along a straight line parallel to the axis on the other side of the filament, outside the tube. This design, which is well adapted to photocopying technology, is not suited, however, to precise positioning of multiple heating filaments. This design also lacks the utility of a one end only electrical connection design utilized in a heating element. It also lacks the versatility of simple selection of series, or parallel electrical configuration.
Both the ceramic fibre design of U.S. Pat. No. 4,001,622, and the existing gold reflector technology result in unnecessary heating of the wall of the tube proximate to the gold or ceramic layer. Radiation traverses this wall once upon initial emission, and again at reflection thus reducing the rigidity of the tube wall and lowering the thermal efficiency of the system.